RFC: Basic CSV Exporter

I've made a PivotTableRenderModel based on the PivotTable, and used it to export the whole table as a CSV file.

As I plan to base the PivotTable on the PivotTableRenderModel for the output (to remove the here introduced code duplication), I`ve  created very specialized classes for every type of cell/row to output, so that it is very easy to use in the PivotTable.

I expect PivotTableRenderModel to change future when I adapt the PivotTable to use it.

This pull request is more a discussion request. You can pull if you like, or wait till I "finish" the rest.
 - Is the whole approach ok?
 - Is the class/method naming ok? Should the exporter be in its own package?
 - I would like to implement this exporter for the "real thing", i.e. Excel => Is a dependency on Apache POI (http://poi.apache.org/) ok? Otherwise I`ll try to build some interfaces mechanism, so that I can plugin a Excel exporter.

Thanks.